1. Field of the Invention
The invention relates in general to protective relaying methods and apparatus, and more specifically to differential relays for protecting electrical inductive apparatus.
2. Description of the Prior Art
Differential relays are the principal form of fault protection for transformers rated 10 MVA, and above. Summing the instantaneous currents in the transformer zone can provide a very quick trip decision when the transformer experiences a phase-to-ground fault, from the percentage current-differential measuring unit. For example, this unit can provide a trip decision in 3 to 5 ms. If there is no internal fault in the apparatus being protected, the differential current resulting from this summation should be zero. If there is an internal ground fault, the current flow into the apparatus will exceed the flow out, with the difference being called the differential current. The current-differential unit compares the differential current with a predetermined value, such as the largest of the currents being summed, or the sum of the absolute values of the currents being summed, and it makes a trip decision when the differential current exceeds a predetermined percentage of this comparison value.
When a transformer is energized, or when the voltage collapses due to a fault external to the transformer's protective zone and then returns following the clearance of the fault, a transient magnetizing or exciting current will flow. This current, also called the inrush current, reaches peaks which are many times full load current, and it appears to be an internal fault to the differentially connected relays. To prevent tripping of the transformer circuit breakers due to inrush current, differential relays for such applications include a harmonic restraint unit. The inrush current is severely distorted, and it has a large second harmonic component. The harmonic restraint unit detects inrush by comparing the fundamental or 60 Hz component with the second harmonic or 120 Hz component. The harmonic restraint unit prevents a trip signal from being sent to a circuit breaker when the second harmonic component exceeds a predetermined percentage of the fundamental. These components are separated by filtering, however, such as shown in U.S. Pat. No. 3,414,772, and the response time of the differential relay with harmonic restraint is increased to at least a full power cycle, such as 17-24 ms. This is true whether the filtering is performed with inductors and capacitors, which introduce certain time delays following the onset of differential current, or digitally with a filtering algorithm. In the latter instance, a full cycle of data is required to accurately extract the different components, and to make the ratio comparison.